This invention relates to gaskets for the joinder of pipes, particularly to features provided in Such a gasket for centering the gasket in a flanged pipe joint.
It is well known to be desirable, in fluid piping systems, to provide a smooth fluid path. Non-smooth surfaces increase flow resistance and trap fluid and particulate matter which would otherwise be easily flushed out of the system. This is particularly important when carrying ultrapure materials. Protrusions and recesses alike trap bacteria or contaminants which may be released unexpectedly and undesirably at later times. In ultra-pure piping systems, contaminants in the parts per billion range may be unacceptable. Such low levels of contamination generally require extremely small surface irregularities in pipe. Concern for this can be so great that, for example, the piping pore size may be regularly inspected using scanning electron microscopy and the pipe joint rejected if the pore size is greater than a maximum allowable value. On account of the fitting together of two different sections of pipe, however, the pipe joint makes an important contribution to the problem of contaminant trapping. Moreover, the quality of this fitting cannot easily be inspected.
In particular, sections of pipe are joined with gaskets. This is because pipe flanges are formed of relatively rigid materials so that they may carry tightening forces. Pipe flange surfaces are not completely smooth or flat, and pipes are not perfectly aligned. Therefore, pipe flanges may not generally be joined in a leak-proof manner without the aid of the relatively compliant, mediating gasket. Within limits, the gasket conforms to the surface quality and angular presentation of the flange surfaces to accommodate any mis-match therebetween, to seal them.
The gasket ideally has an aperture conforming to the inner periphery of the pipe flanges, and extends radially outwardly along the interior surfaces of the pipe flanges. Typical prior art gaskets include holes through which bolts for tightening the joint are passed. The holes are larger than the bolts to ease their installation. However, the holes also provide for locating the gasket with respect to the inner periphery of the pipe flanges. It is not possible, therefore, to position the aperture to be precisely congruent with the inner periphery of the flanges.
The aperture is, typically, made "on-size", i.e., so that the diameter of the aperture is about equal to the diameter of the inner periphery of the flanges. Since registration of the aperture with the inner periphery of the flanges is not perfect, however, the gasket generally protrudes into the stream of fluid flow in some places, and recesses from the inner surfaces of the pipe flanges in other places. Moreover, such traps cannot be avoided by making the gasket over or under-sized.
Accordingly, there is a need for a gasket having centering features that provide for improved centering the gasket in a flanged pipe joint, to decrease flow resistance and the propensity to trap fluid and particulate matter.